| Artikel Dua
http://irc.nrc-cnrc.gc.ca/cbd/cbd148e.html
CBD-148. Foundation Movements
C.B.
Crawford
In designing a structure it is commonly assumed that the foundation
will not move. Correspondingly, if cracks appear in the structure it is
assumed that the foundation did move and that this is the sole cause of
cracking. Neither assumption is correct. To assess the influence of the
foundation on cracking it is necessary to take into account the nature
and magnitude of foundation movements and to understand why and how they
occur.
Within reason, there is less concern for the total settlement of a
building than for differential settlement. The Palace of Fine Arts in
Mexico City, for example, has sunk more than 10 feet into the ground
since it was built 60 years ago and the most noticeable effect is that
the grand stone stairway has disappeared and the entrance is now at
street level. Classical failures like this seldom occur today.
Foundation failures are now quite rare, due largely to improved
understanding of the properties of soil and rock materials.
Nevertheless, detrimental movements do occur occasionally and it is the
purpose of this Digest to explore the possible causes.
Soils and bedrock are similar to other building materials in that
they deform under load, but unlike them they must be used as they appear
in nature; they cannot be controlled by a manufacturing process. Except
for special cases bedrock can be excluded from consideration because it
is normally an adequate foundation material. Soils, on the other hand,
are often stressed to their limit by foundation loads.
Foundation Stresses
The prediction of foundation movement is based on knowledge of how
foundation loads are transferred to the ground and how earth or rock
materials respond to resulting increases in stress. There are too many
variables for these predictions to be precise, but for most situations
they are adequate.
Consider first how stresses are transferred to the ground under a
large and a small footing, each carrying the same unit pressure (Figure
1). The curved lines under the footings are lines of equal increase in
stress due to footing load. This is often called the "bulb of pressure."
Note that the deepest line, indicating a stress increase equal to 10 per
cent of the applied load, extends to a depth twice the width of the
footing. If a series of narrow footings is installed close together the
bulbs of pressure intersect and the influence on the ground is deeper
than for an isolated footing. When piles are used, foundation loads are
carried to deeper strata. If the piles are long in relation to the width
of the building, the effect is much greater than if the piles are
relatively short. The bulb of pressure concept is used to determine the
depth to which foundation soils must be explored.
Figure 1. Lines of equal vertical stress caused by surface loads.
Examples of Settlement
As building loads are applied to the ground an "immediate" settlement
occurs as a result of instantaneous compression of the soil. Most
immediate settlement may be accommodated within the structure as it is
built, and fortunately much of the differential movement occurs at this
stage. Under certain conditions, however, fine-grained soils will
continue to compress under constant load for many years. This long-term
compression is called "consolidation" settlement and is caused by the
squeezing out of water from the pores in the clay.
Differential settlement occurs for a number of reasons:
-
local variations
in soil compressibility,
-
variation in
thickness of compressible soil,
-
differences in
footing sizes and pressures,
-
variation in
applied loads,
-
overlapping
stresses,
-
differences in
depth of embedment of footings.
A classic example of consolidation settlement is occurring under the
Empress Hotel in Victoria, B.C. This building is founded on 50-foot
piles which rest on gravel at one end but penetrate only to the middle
of a compressible clay layer at the other. Although the maximum
settlement at the deep clay end is more than 30 inches, the damage is
not especially serious because the building has tended to tilt on a
plane. Fortunately, level observations have been taken annually since
1912, shortly after the Empress Hotel was built, so that it has been
possible to reconstruct the loading and settlement history from the
beginning of construction. This shows that settlement occurred rapidly
during the first five years and continues slowly 65 years after
construction.
If foundation loads vary, the differential settlement can be more
serious even when sub-soils are relatively uniform. The National Museum
Building in Ottawa is an example of such a situation. This massive
structure has a complex footing system on two levels, with bearing
pressures varying from less than 1 ton per sq ft to more than 4 tons per
sq ft. The differential settlement became so serious five years after
the building was opened that the tower structure over the main entrance
had to be removed in 1915 to prevent collapse. The estimated total
settlement varied from zero where the bearing pressures are small to 1.6
feet under the tower. A few years ago a section of original flooring was
removed, revealing that as much as ½ foot of settlement had occurred
during construction so that the total differential settlement at footing
level is probably more than 2 feet. This has caused considerable damage
to some interior partitions, but owing to the nature of the framing the
basic structure is sound.
Another example will illustrate how modern foundation practice
permits successful building on even very poor subsoils. An industrial
plant on the south shore of the St. Lawrence River at Varennes, 20 miles
downstream from Montreal, rests on a 2.5-foot thick reinforced concrete
mat over 100 feet of compressible clay. The mat foundation was
integrated with pile foundations for special machinery. Mat pressure
varies from 700 to 1,700 psf over the 100- by 300-ft building area.
Settlement varies from more than 6 inches under the heavily loaded area
to about 2 inches under the lightly loaded area. Most of the settlement
occurred during the one-year construction period ending in 1957, but
another 2 inches occurred in the six years following construction. This
degree of differential settlement was perfectly acceptable to the owners
on an economic basis and an extension begun in 1961 was built in the
same way. The buildings were designed with sufficient flexibility to
allow the differential settlement to pass virtually unnoticed.
A second example from the same general soil area has not turned out
so happily. In this case a large warehouse connected to a special
facility on piles was founded on a 3-foot sand fill over compressible
clay. The designers did not realize that the sand fill added more load
to the ground than did the structure. The result has been more than 1 ½
feet of unanticipated differential settlement, accompanied by
considerable damage and operating difficulties.
In most areas of Canada the differential settlement of an important
building can be limited to a fraction of an inch. The 26-storey CN Tower
in Edmonton is a good example of the foundation performance that can be
expected when modern technology is applied. This reinforced concrete
structure, founded on spread footings resting on a sandy or silty clay
till at depths of 22 to 26 feet, had a maximum settlement after 6 years
of just over 1 inch, with a differential of less than ½ inch; 80 per
cent of the settlement occurred during construction. The risk of
detrimental settlements appears to be much greater in more modest
structures such as 10- or 15-storey apartment blocks where owners may
often be inclined to skimp on foundation investigation and design.
Types of Settlement
There are three basic types of settlement: uniform settlement, tilt,
and non-uniform settlement (Figure 2). Uniform settlement and tilt
(within reason) do not greatly affect a structure, but resulting
movements may cause serious problems with services and appendages such
as water mains and connecting tunnels. Non-uniform settlement is
characterized by angular distortion and may cause cracks or even
structural failure. The degree of angular distortion is indicated by the
ratio of differential settlement to distance between supports,  /L.
Laboratory tests and field experience have given reasonable correlations
between angular distortion and damage for various kinds of construction
ranging from 1/750, where difficulties with sensitive machinery may
occur, to 1/150 where structural damage is to be feared.
Figure 2. Types of settlement.
The amount of settlement that a building can tolerate, the
"allowable" settlement, depends on its size, type and intended use. The
allowable settlement for the plant at Varennes, for example, would not
be permitted in a city hall. For practical reasons, the amount of
settlement that would be tolerated under the difficult conditions of
Mexico City is greater than would be tolerated in any Canadian city.
Swelling and Shrinking Soils
So far the discussion has been confined to foundation movements
caused by compression of subsoil due to loading. Large movements can
also occur from shrinking or swelling of clay subsoil resulting from
stresses unrelated to foundation pressure.
Fine grained clay soils may be subjected to extremely high stresses
due to air drying or vegetation. Shrinkage may take place throughout the
full depth of rooting and the depth of the active layer depends on both
climatic conditions and vegetation. Drought-resistant vegetation growing
in semi-arid climates may have roots extending deeper than 20 feet.
Because tree roots are extremely efficient in extracting soil moisture,
both the depth and rate of shrinkage can be greatly accelerated in soils
supporting such vegetation. Some soils will swell back to their original
volume when they are rewetted. The movement of a footing up and down,
therefore, depends on the condition of the soil at the time of
construction and on subsequent wetting and drying. Usually the worst
heaving conditions develop in soils that have been previously desiccated
by heavy vegetation or extreme aridity and then subjected to greatly
increased moisture conditions as a result of construction and irrigation
practices.
Although shrinking and swelling usually affect only shallow, lightly
loaded footings they can cause considerable damage even to large
buildings. Swelling soils are common on the Prairies. In Winnipeg, an
extension to a church was built over an area in which the trees were cut
down immediately before construction. The floor slab for the addition,
which rested directly on the ground, was heaved 6 inches in two years,
giving an annoying discontinuity between the old and new sections.
Although the main structure was supported on piles, there was severe
damage to partitions and finish in the basement, with some distortion
transmitted to the superstructure above grade.
It is common in many of the heavy clay regions of Canada for shallow
foundations to move up or down by several inches. In these regions the
problem can be overcome by designing a rigid structure to reduce
differential movement or by providing a deeper foundation (usually short
piles) to carry the structure and allowing space under floors for soil
movements.
Other Causes
There are several other causes of foundation movement worthy of
mention. Freezing of the ground is a problem in most parts of Canada.
When fine-grained soils such as silts and clays or even dirty sands and
gravels freeze, water is drawn up from the water table to freeze into
discrete lenses and cause a volume expansion. Usually the normal winter
heating keeps frost away from footings, but the possibility of frost
heaving under attached garages or depressed driveways is often forgotten
and results in considerable damage to light structures. Frost action is
also a hazard during construction. A large building under construction
in Ottawa was badly damaged during a cold winter period. Basement floor
slabs were heaved and cracked and the movements extended up through the
structure, causing distortion and cracks in partition walls. In Northern
Canada there are many cases of serious distortions of heated buildings
due to the thawing of ice in the permafrost under them.
Even bedrock is not always reliable. The basement floor of a downtown
building in Ottawa began to heave mysteriously several years after
construction. Research revealed that the pyrite in the shale bedrock was
being converted by oxidation and bacterial action to gypsum and other
sulphate materials to cause swelling. Parts of the floor heaved as much
as 4 inches in five or six years, although only minor structural
distortion was caused before the swelling was arrested by chemical
treatment.
It may be concluded from this review that foundation movements always
occur. The important point to remember is that foundation performance
can be satisfactorily predicted. For simple foundations on good sites
this can be done with a modest expenditure. At poor sites, or where
foundations are complex, an extensive and possibly expensive
investigation is required. The designer has to compromise between his
desire for zero movement and the owner's desire for the cheapest
foundation. On the one hand it may be advantageous to accept rather
large differential movements and to design joints to accommodate them.
On the other hand he may be able to demand a foundation sufficiently
stable to allow deflections to be ignored for purposes of joint design,
permitting joints provided for other purposes to accommodate small
deflections due to foundation movement. Because of the interaction of
the soil and the structure, the advantages of having the foundation
engineer and structural engineer work together are very clear.
Originally published April 1972.
|
